Story highlights

An MIT police officer was killed, and a transit police officer was injured during the chase

Cambridge and Watertown are hot spots in investigation as police seek second suspect

In Cambridge, Massachusetts, FBI agents -- guns drawn -- walked down the street near the home where the Boston Marathon bombing suspects lived.

Emergency vehicles lined the street, roads were cordoned off, and a suspicious red vehicle was towed from the scene.

Mark Santos, 29, a doctorate student at the Massachusetts Institute of Technology, said it's still sinking in that the suspects, whom he didn't personally know, lived directly across Norfolk Street.

"We were that close," said Santos, who has lived there since September. "It's not something you expect."

His roommate, Diana Jorge, 26, said she's been confused ever since police ordered her out of her home.

"I'm a bit worried. I never thought that this could happen here in Boston," she said.

Many residents in the area awoke to federal agents with machine guns banging on their doors and the commands: "Get out! Get out!"

Despite the calls for evacuation, some residents sat on sidewalks near a perimeter of crime scene tape. Others defied the commands to stay indoors and gathered to gawk as state and federal officers -- some in camouflage, most in flak vests -- canvassed the neighborhood.

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Four miles west, in the suburb of Watertown, many residents remained on lockdown, stuck in their homes after a car chase led the Boston Marathon suspects and a cadre of police into their neighborhood.

Chaos ensued just hours after authorities released images of the suspects.

It was about 10 p.m. -- close to bedtime for many people, if not past it -- when police received a report that a robbery was under way at a convenience store near the MIT campus in Cambridge.

The robbery was the first in a series of crimes that unfolded overnight and well into the next morning, putting the city, suburbs and one of the country's premier universities on lockdown at various points through the night. Residents were told to stay inside, away from windows, as city services came to a halt.

About 20 minutes after the robbery, an MIT police officer was fatally shot while responding to a disturbance at Vassar and Main streets, amid several of the school's research facilities.

MIT was quick to warn its students of the mayhem on its Twitter feed and emergency services website.

"Gunshots were reported near Building 32 (Stata) which is currently surrounded by responding agencies. The area is cordoned off. Please stay clear of area until further notice," the university posted, asking students to stay away from one of the university's busiest buildings.

Police arrived on the scene to find the campus police officer in his car with multiple gunshot wounds. They took him to Massachusetts General Hospital, but it was too late. Patrol officer Sean Collier, 26, of Somerville, who joined the force in January 2012, was pronounced dead. Police believe the bombing suspects were responsible for the shooting.

"Sean was one of these guys who really looked at police work as a calling," MIT Police Chief John DiFava said in a statement. "He was born to be a police officer."

Shortly after the shooting came another report of violence: Two men -- whom police now say were brothers living in Cambridge -- hijacked a vehicle at gunpoint in Cambridge, telling the driver that they were the marathon bombers, a law enforcement source told CNN's Joe Johns.

At some point, at a gas station on Memorial Drive along the Charles River, the driver escaped uninjured, Cambridge police reported.

Police, who were tracking the vehicle using its built-in GPS system, picked up the chase in Watertown, a town of 31,000 just west of Boston. The pursuit went into a residential neighborhood, with the suspects throwing explosives at the police, officers said.

"The suspects and police also exchanged gunfire in the area of Dexter and Laurel streets. During this pursuit, an (Massachusetts Bay Transportation Authority) Police officer was seriously injured and transported to the hospital," Boston police said in a release.

Richard H. Donohue Jr., 33, was shot and wounded in the incident, a transit police spokesman said. CNN affiliate WBZ reported he has been with the department for three years. He was taken to Mount Auburn Hospital, WBZ said. The officer's condition was not immediately known.

Ultimately, one bomber -- later identified as Tamerlan Tsarnaev, 26 -- got out of the car. Police shot him, and his brother ran over him as he drove away, according to the law enforcement source.

Tamerlan Tsarnaev was critically injured. He had bullet wounds and an injury from an explosion. He was wearing explosives and had an explosive trigger, a source told CNN. He later died, while authorities say his brother, Dzhokar Tsarnaev, 19, escaped.

After a hellish week in Boston, residents probably didn't need to be reminded that he was believed armed and dangerous.

Warnings to Watertown residents followed. First, Boston police tweeted, "There is an active incident ongoing in Watertown. Residents in that area are advised to remain in their homes. More details when available."

Then, a warning by state police that the search was intensifying: "Police will be going door by door, street by street, in and around Watertown. Police will be clearly identified. It is a fluid situation."

Officers locked down the streets of a Watertown neighborhood -- the intersection of Dexter and Laurel streets lies in a heavily residential area -- after isolating the vehicle. SWAT members arrived on the scene, and police in full body armor carrying assault rifles ran down the streets, according to CNN affiliate WCVB.

Homeland Security Investigations agents were among those responding, a department spokesman said.

Police requested that residents in the area turn off their mobile phones, saying they believe the devices were used to detonate the Boston Marathon explosives. They went door to door, leaving an already-on-edge suburb even more anxious.

Chris Howes, who asked local police via Twitter whether they had any advice for residents who lived on Norfolk, also wrote on Twitter, "This is scary as f***." Asked via Twitter whether he could provide more information, he tweeted, "don't really have any info. CPD told us to stay indoors so we just hear lots of sirens outside."

As news emerged that the suspects lived with their parents in the Norfolk Street area, nearby resident Amy McConnell tweeted, "Just so weird to think those guys have been walking around my neighborhood all week." She also declined to speak to CNN.

At the corner of Norfolk and Cambridge streets, police tape lined the road, blocking off Norfolk a few houses down from the white clapboard home where the suspects reportedly lived with their parents. Police and FBI buzzed around the home.

Joey Barbaso, 50, has lived in the neighborhood since he was 5. He didn't know the suspects, had never seen them.

The construction worker's pants were worn and stained with paint. The neighborhood, nestled between Harvard and MIT, is a mix of working class and college students, he said, standing in the doorway of a shop.

"You never know who you're living next to," he said. "I think it's nuts. What's this world coming to? I tell you, the world's getting screwed up more and more and more."

Meanwhile, the entire city was shutting down. The transit authority said all modes of transport -- including rail, subway, buses and ferries -- would be suspended until further notice. Taxi service in Boston was suspended.

MIT and Harvard canceled classes, as did other local colleges Boston Public Schools.

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While police requested residents of Boston and all its suburbs remain at home, the Massachusetts State Police specifically singled out Boston, Watertown, Cambridge, Allston-Brighton, Belmont and Newtown. The governor's office added Waltham to the list of places where people should "stay indoors with your doors locked."

Later, the Boston Police Department announced that "all vehicle traffic" in Watertown was suspended and asked that businesses remain closed. The transit authority sent buses to evacuate residents, while bomb squads combed the area.

Cambridge police -- already slammed -- announced on Twitter that they had fielded more than a dozen calls about suspicious packages in the city. All were cleared without incident.

As dawn came, police announced they were conducting a controlled detonation near Kenmore Square, across the river from MIT. They also released a vehicle description: "Police seeking MA Plate: 316-ES9, '99 Honda CRV, Color - Gray. Possible suspect car. Do not approach."

The car was later located in Cambridge, but Dzhokar Tsarnaev was not found.

Another controlled detonation was announced later in the day, as well as another lookout, this one for a green 1999 Honda sedan.

As the situations in Cambridge and Watertown continued to unfold well into the afternoon, many residents woke to a city in turmoil, though some aspects of life were returning to normal, including taxi service and trash pickup. Still, residents were nervous.

Tweeted @BarryGagne of Watertown, "I'm seriously scared right now. Way to close to my house. (2blcks)Afraid of explosives. Everything. Be safe people #watertown #bostonstrong"

Back in Cambridge, authorities erected a blue tent-like tarp outside the suspects' home. Much of the home's contents was placed beneath it, and reporters, who had been kept a block away from the scene, were allowed to have a closer look.

Though across-the-street neighbors Santos and Jorge didn't know the suspects, they had succinct sentiments regarding their fate: They hope the remaining suspect meets the same end as his brother.

"I hope he's found dead," Santos said. "I don't think we should pay U.S. taxpayer dollars to give him a fair trial. Nah."